Respiratory disturbances are associated with a number of pathological conditions. Cheyne-Stokes respiration is the waxing and waning of respiration associated with congestive heart failure. Kussmaul breathing is rapid deep breathing associated with diabetic ketoacidosis. Central or obstructive forms of sleep apnea are prevalent in both normal and heart failure populations. Detection of respiratory disturbances, such as Cheyne-Stokes respiration, Kussmaul breathing, apnea or hypopnea episodes associated with obstructive or central sleep apnea or other disordered breathing, may be useful in monitoring a patient's disease status, selecting treatment and monitoring its effectiveness.
Respiratory disturbances in the form of sleep-related disordered breathing may often go undetected in patients suffering from heart failure or sleep apnea. Nocturnal Cheyne-Stokes respiration, a form of central sleep apnea, occurs frequently in patients with chronic heart failure. The presence of sleep apnea significantly worsens the prognosis for a heart failure patient. A method for determining the cardiac condition of a patient by a cardiac monitor using the variability of a respiration parameter is generally disclosed in U.S. Pat. No. 6,454,719 issued to Greenhut, incorporated herein by reference in its entirety. Characteristics of periodic breathing patterns, such as hyperpnea length, apnea length, and periodic breathing cycle length, are correlated to circulatory delay time, which is inversely correlated with cardiac output. Therefore, recognizing and monitoring the presence of disordered breathing in heart failure patients could provide useful diagnostic and prognostic information. Moreover, detecting respiratory disturbances and extracting specific parameters related to cardiac function could provide valuable information for assessing a patient's cardiac condition and optimizing therapeutic interventions.
A standard diagnostic approach for sleep apnea includes polysomnography, which requires the patient to stay overnight in a hospital for observation, in addition to medical history and screening questionnaires. Polysomnography involves monitoring of multiple parameters including electroencephalography, electromyography, electrocardiography, oximetry, airflow, respiratory effort, snoring, body position and blood pressure. Polysomnography measures a patient's respiratory patterns during a single sleeping period and is expensive and inconvenient for the patient. Furthermore, a physician must actively prescribe the sleep study and therefore must already suspect a sleep-related breathing disorder. Chronic monitoring of respiratory disturbances as an alternative to polysomnography, particularly in heart failure patients who have increased risk of morbidity in the presence of sleep apnea, is desirable for detecting unrecognized and unsuspected sleep-related disordered breathing.
Chronic monitoring of respiratory disturbances is also desirable in diabetic patients. Diabetic ketoacidosis may be the first symptom to appear in a person with Type I diabetes. Diabetic ketoacidosis develops when blood is more acidic than body tissues due to the accumulation of ketones in the blood when body fat is metabolized for energy in place of glucose reserves when insulin is not available. Persons having Type II diabetes usually develop ketoacidosis only under conditions of severe stress. Recurrent episodes of ketoacidosis in diabetic persons are generally the result of poor compliance with dietary restrictions or self-administered treatments. Kussmaul breathing is a common symptom of ketoacidosis. Therefore early detection and monitoring of Kussmaul breathing in diabetic patients may be valuable in the effective control of diabetes. Kussmaul breathing is typically characterized by relatively deep breathing Respiratory monitoring may be a preferred method for monitoring diabetic status in combination with or in place of periodically measuring blood glucose, which requires the use of hollow needles with associated pain and risks of infection.
Respiration may be measured directly using, for example, external breathing masks. Breathing masks, however, are generally not well tolerated by patients for extended periods of time. A system having one or more sensors disposed at or near the surface of a patient's body for monitoring physiological variables of a patient in real time, including respiratory sounds for the detection of abnormal breathing patterns, is generally disclosed in U.S. Pat. No. 5,738,102 issued to Lemelson.
A system for monitoring respiration using an implantable sensor would allow chronic, ambulatory monitoring for respiratory disturbances. Impedance measurements made using implanted electrodes may be used for detecting changes in thoracic impedance associated with changing lung volume during inspiration and expiration. Tidal volume and respiration rate may be determined from the measured impedance. Minute ventilation may then be calculated from the extracted tidal volume and respiration rate. Normal changes in minute ventilation in response to exercise are measured using impedance sensing in some cardiac pacemakers in order to provide a sensor-indicated pacing rate for rate-responsive cardiac pacing. See, for example, U.S. Pat. No. 4,901,725 issued to Nappholz.
Respiratory signals may be extracted from other physiological signals that can be obtained from implantable sensors. Other physiological signals, such as subcutaneous ECG, cardiac electrogram (EGM), blood pressure, and heart sound signals, typically contain cyclical amplitude changes due to the influence of the respiratory cycle. Pulsus paradoxus refers to a decrease in arterial blood pressure that occurs during inspiration. A device for measuring pulsus paradoxus for assessing and monitoring patients with respiratory disease is generally disclosed in U.S. Pat. No. 6,325,761 issued to Jay, incorporated herein by reference in its entirety. A method for computing tidal volume as a function of extracted blood pressure information indicative of the change in blood pressure that occurs over a respiratory cycle is generally disclosed in U.S. Pat. No. 5,980,463, issued to Brockway, et al., incorporated herein by reference in its entirety. Implantable cardiac rhythm management or cardiac monitoring devices may sense ECG, EGM, blood pressure and/or other physiological signals that are variable due to the influence of respiration. Thus, an opportunity exists for detecting changes in respiration rate using physiological signals already sensed by an implantable device without having to introduce new sensors.
It is desirable to provide a system and method for detecting and monitoring episodes of respiratory disturbances based on a physiological signal sensed by an implantable sensor, which disturbances may be associated with a particular pathological condition. Monitoring of respiratory disturbances may be used for diagnostic or prognostic purposes and may further be used for triggering other types of patient monitoring or the delivery of a desired therapy.